New console retargeting modularity

Today's work involved refactoring the console support code to support retargeting. I had previously done some basic retargeting work at the VHDL level, but I want to keep the VHDL code as abstract and generic as possible, so the retargeting stuff has to move down to the native level. The console functions are now simple getc()/putc() generic functions, and everything involving switching between (for example) the Ncurses and libc (and someday raw text-buffer/IO port) console targets will be done in native code and will be selected at compile time based on the overall system target.